Bile secretion is a major hepatic function, vital to the survival of man, yet often impaired in diseases of the liver resulting in cholestasis. Our previous studies of bile formation and cholestasis carried out in the past six years (#AM17153 - 1974-1978 and #AM25636 -1979-1980), have led to the proposal of a new model of bile secretory function. Three basic tenets of this model will be tested in this proposal: 1) Na ion-, K ion-ATPase is a basolateral enzyme which maintains a sodium gradient that provides the driving force for the coupled entry of bile acids and other anions. 2) Bile acids and micellar components are excreted into the bile canaliculus by a process involving vesicle transport. 3) The structural and permeability properties of the junctional complexes between hepatocytes help regulate biliary secretion and may contribute to cholestasis. The methods which we will use to examine these three tenets include: A. Histochemical and biochemical localization of Na ion-, K ion-ATPase in isolated rat liver plasma membranes and the use of this membrane marker to facilitate the separation of basolateral and apical (canalicular) membranes for studies of the transport functions of these two membrane domains. B. The study of coupled sodium and anion transport in isolated rat hepatocytes and membrane vesicles. C. Purification and content analysis of Golgi and pericanalicular membrane vesicles. D. Detailed study of the freeze fracture replica anatomy of junctional complexes between hepatocytes in choleretic and cholestatic models and their relationship to changes in biliary permeability to ionic lanthanum chloride and the clearances of insulin, sucrose and ferrocyanide in rat and skate liver systems.